Pick Parts, Build Your PC, Compare and Share – PCPartPicker
This is not something new to most I’m sure, but for me it was a cool fine, almost like a virtual Lego builder. This site will help you assemble a computer from compatible parts and see the best prices.
Once you have a plan, you can export your list and post it to http://www.reddit.com/r/buildapc/ where a community will refine, advise, critique and encourage.
Warning, this is addictive! I’m playing around with plans for a Linux machine with an SSD for HD and foolish amounts of RAM, which I’ll use directly or via VNC. Btw, it’s super cheap and you get to cut corners or sup it up as you’d like.