When using the OmniWeb browser, sometimes it'll get really unresonsive and crummy to use. Here's one way to fix it:

1. Quit OmniWeb.
2. Go to ~/Library/Application Support/OmniWeb 5 in Finder. In my case, this is /Users/bkerley/Library/Application Support/OmniWeb 5
3. Drag HistoryIndex.ox and the .log and .lock file (if they exist) into the trash.
4. Re-start OmniWeb.

My guess (an educated guess, thanks to Activity Monitor's Sample feature) is that this index of history items gets corrupted, and OmniWeb gets stuck in a tight loop trying to read or make sense of it. The index re-generates from History.plist pretty quickly on modern systems, so deleting it is no real loss.