What is the bandwidth required?
For high bandwidths see:
Look at www.bitscope.com (COOL DESIGN BUT COSTLY) **broken link removed** (another good design but components are hard to get)
Do a web search for Winscope (written at Moscow State University), or better yet, go to www.dazyweblabs.com for a whole slew of free downloads; oscilloscope, frequency meters, oscillators, etc. - all PC-based through your sound card, all free. Enjoy!
Bistscope is the cheapest full-featured PC DSO I've found. It has the highest bandwidth along with loads of other features. You also have plenty of different softwares to choose from. If you compare bitscope with other commercially available PC scopes, you'll find that it is many times cheaper than other ones.