Burst!Media (http://www.burstmedia.com) - Burst!Media is one of the oldest ad networks and it also acts as an advertising agency and can get targeted ads for your site. Basically this network looks for smaller niche content sites that it can group into categories and then offer those sites as a group. So if you don't get enough traffic to get big advertisers, Burst can pair you up with similar sites so that your "package" is more attractive to large advertisers. By and large Burst! gets fairly good ad rates, especially on targeted advertising. They also have fairly low traffic requirements for joining, requiring just 5,000 monthly impressions. However they have fairly strict topic requirements and if you run a general audience or popular culture type site they will likely reject you. Burst supports 468x80 banners, 160x600 skyscrapers, leaderboards, popups, popunders, and cubes. Burst's popunder controls aren't as good as other companies (nor are their rates). You can only limit how many pops a person views per session, not per day or every 12 hours like other networks allow.
Tribal Fusion (http://www.tribalfusion.com) - Tribal Fusion hasn't been around as long as Burst!Media, so they have less experience or contacts in the business world. They also have less pull with large advertisers. Tribal Fusion has higher traffic requirements than Burst!Media, they require 1000 uniques per day, which could mean on average around 150,000 impressions per month. They support 468x60 banners, 728x90 banners, popups, popunders, and skyscrapers on select sites. Tribal Fusion's pops are limited to one view per person every 12 hours. Tribal Fusion takes a 45% commission on ad sales. I have never heard anyone complain about Tribal Fusion. Good word of mouth has made them increasingly popular with webmasters. I would definitely recommend giving them a try if you have enough traffic - although they shouldn't be used as your only provider. Right now I would personally recommend them for your primary provider, especially if you have room for 728x90 banners.
FastClick (http://www.fastclick.com) - What really sets Fastclick apart is its ad serving technology. They have a very flexible, very modular, ad serving system and they offer a plethora of member features. Many webmasters use Fastclick as their primary provider because with all the management features it is very easy to manage the ads you send to other, secondary networks. Fastclick also has a low requirement of only 3,000 impressions per month. The only real downside to Fastclick is they can't bring in really good ad rates. However for general audience advertisements they're one of the best. I highly recommend Fastclick to everyone. Their banners and skyscrapers are not very good, but their popunders are just about the best. You can join them and run only their popunders and you'll do fine. The key is to select almost every popunder they offer and them limit them to one per person per day or one per person every 12 hours. As for banners and skyscrapers - just use them as your last tier provider.
AdSense (http://www.google.com/adsense/) - AdSense is a great program that was launched by Google the summer of 2003. What it does is it provides a way to connect Google's thousands of AdWords advertisers with thousands of small publishers. For the first time small advertisers are able to easily advertise with small publishers. This program is strictly CPC and Google does not disclose it's commission rates, even to AdSense members. Despite being CPC the program can perform remarkably well on many types of sites depending on subject matter. If you deal with subjects that involve high bids in Google's AdWords program then you could make a killing with AdSense. You can display Adsense ads in leaderboard (728x90), banner (468x60), or skyscraper (120x600) formats. Additionally if you can send AdSense more than 20 million impressions a month you can negoitiate a long term CPM contract with them. AdSense is highly recommended for sites that are a good content fit, and there are no traffic requirements so even the smallest niche sites can profit with AdSense.
ValueClick (http://www.valueclick.com) - ValueClick started out as a CPC only network, though now they serve all types of ads. They are also not an advertising agency and will not work to get targeted ads for your site. They're about as old as Burst!, so they are well established and have a good reputation among website publishers. ValueClick offers nearly every ad size under the sun, far too many to list here. This makes them a very good choice because of the flexibility of having so many choices. ValueClick requires 15,000 page views a month and instead of having publicly available information on commissions, they simply figure in how much they will take off the top before informing you of how much the ad will pay. Because of their flexibility, ValueClick makes a good choice for a secondary provider. However, while they are consistent in what they bring in, they don't bring in very much. They are typically a tertiary provider for many webmasters.
24/7 Real Media(http://www.247realmedia.com)- 24/7 is one of the networks that caters to larger publishers. Their minimum traffic requirement is 500,000 impressions a month, though I am told that this is not a hard and fast rule and they'll sign smaller sites if their traffic is very targetted. Their standard media sizes are 468x60, 120x600, 728x90, 160x600, pop-up, pop-unders, and text links, though for specific site campaigns they can offer other creative types. Currently 247media takes 50% commmission on advertising sold. They will also charge a default serving fee if your traffic is not atleast 80% US.
FocusIN(http://www.focusin.com) - FocusIN is primarily a CPC network that groups sites together in categories like Burst/Tribal Fusion to achieve semi-targeted ads. Their rates are relatively low in comparison to the others and they only offer banners.
DoubleClick(http://www.doubleclick.com) - DoubleClick used to be a large advertising network, similar to 24/7 Real Media. However in July of 2002 they sold their media business to L90 and now only provide ad serving technology, and not any ad sales. I only mention them here because you may hear of them in conversation.
Advertising.com(http://www.advertising.com) - Once a very popular and highly recommended CPC network Advertising.com has severely degraded over the past one or two years. I have heard nothing but complaints about them for quite awhile. In light of that I have to recommend you stay away from them.
Sprinks(http://sprinks.about.com/da/distind.htm) - Once known as "Luna Network" Sprinks is run by About.com. Originally they looked very promising, the deal was you would have to be exclusive with them and they would provide very high paying advertisements, along with required links back to About.com. They would also provide links to your site from About.com. It looked like a match made in heaven. However it ultimately failed, many publishers got very upset because About.com changed the rules a few times, and now its nothing more than a cheap CPC network.
Gorilla Nation(http://www.gorillanation.com) - Gorilla Nation is a small network, (150 sites) of entertainment and technology websites. They are very selective and you need at least 500,000 monthly impressions to apply.
There are more advertising networks out there, and almost monthly there is a new one, or an old one shuts down. The ones listed above are generally more popular (or notorious), and you should be able to find representation from one of them.
What about selling your own advertising? I don't recommend it for beginners. When you're dealing directly with the advertiser you take on additional responsibility and liability. You could run into legal issues involving payment or delivery, and its just hard for a new site to find advertisers. Until you get to a decent size and have some more experience you'll be better off going with an ad network.