i've been using ClipX on windows for quite some time, but it seems the latest version (which haven't been updated for quite some time) has some bugs, or perhaps it's windows 7 with something installed or used at the moment.
from time to time it did not catch all the "ctrl + c" events from text editors, so a couple of months ago i've moved to Ditto, which has much more features, yet at the same time can remain quite simple when in action. it can store a very large number of entries in a database that can be cleared any time and is still quite responsive when retrieving something (perhaps slightly slower that ClipX).
on the linux side of things, the only one i've tried is parcellite, which apparently to a majority of users is the best one out there.
perhaps future OS's will have native implementations of clipboard (history) managers pre-bundled.