Features:
Reliable tracking software, Easy to use web based interface, Pay per performance (Pay per Sale, Lead and/or Click, Monitor individual affiliate performance, Easy to generate commission and sales reports, Ad creative management - organize your links by websites / products / types etc, Hardware-free, 24 hour support staff.

Recommendation: AffiliateRunner software is about the easiest and most intuitive I have seen. The tools are powerful yet easy to manipulate. The software has everything you need to manage an affiliate program with ease. Check out the live demo video on their home page. You'll see.

Statement from AffiliateRunner:
"This reliable and advanced software tracks visitors led from affiliate websites and provides you with data and statistics to efficiently control and manage your own advertising campaign. 

Affiliate Recruiting
We can tackle the hard part and find affiliate partners for your site, even if you are using another affiliate software provider! 
 
Affiliate Program Management
Not familiar with running affiliate campaigns? Let us manage everything while you relax and watch traffic and sales soar."

For a Demo:
You can view a demo of AffiliateRunner here.