Plimus - Very similar to clickbank but with different offers, easy to get accepted.
Markethealth - Focuses more on health/wellbeing based offers, mostly physical products, easy to get accepted.
Linkshare - Easy to get accepted, great tracking. However often has lower commission rates because merchants have to pay a lot for the use of their service and technology. Payments often take longer than other networks.
As Seen On PC - Relatively easy to get accepted, but prefers if you can show an example website. Great because they have a lot of physical product offerings with solid payouts, which is good for your newsletter sequence if you are looking for more products to promote in a particular list or on your website.
Commission Junction - Easy to get accepted, commissions rates vary wildly, some are good, others are terrible. The interface is a little tricky, but easy when you get the hang of it. Offers a wide variety of offers, some are really well known. Offers product feeds.
Shareasale - Good ever growing selection of offers, not as large as CJ but on average the commissions are much better. Relatively easy to join and to use.
Neverblueads - They prefer intermediate and advanced affiliates to apply here. It would be better to set up and promote a clickbank or other affiliate product first before applying here so you can show an example of your website(s). Make sure you mention you will be sending a lot of ppc traffic (if you will be) as this is a good selling point to get yourself accepted here. Has great affiliate managers.
Azoogle - Similar to Neverblueads in that they prefer intermediate and advanced affiliates. They run regular affiliate contests to spice things up. I haven't used Azoogle much myself yet but I hear good things about their affiliate managers from other affiliate friends of mine.
Affiliate.com - Similar to Neverblueads in that they prefer intermediate and advanced affiliates. Some good CPA offers here, but not a huge variety. That said, some affiliates are making a killing off their top few offers.
Uniqueleads - Prefer intermediate and advanced affiliates, has some great performing offers and is an up and coming affiliate cpa network.
Now there are hundreds more networks than the ones that I've listed above, but they are ones that either I know others are being hugely successful in, or I have met personally with the networks and know of the quality of their organization and offers.