Have use for the more obscure O'Reilly books, but don't want to buy the books? Subscribe to Safari and have access to pretty much the entire O'Reilly library online (300 volumes now, "the rest later"). At their least expensive plan, you can have access to five books at once for $10/month. It's actually a "point" system, so for $10 you get 5 points that you can spend on books ranging from 1/2 point to 3 points each, with most books 1 point each. You get 10 points for $15. Change which books you have open each month. The full text of the entire library is searchable and browsable. This wouldn't be worth it if you'd only use a few titles, but the library is so large now that it could actually be quite useful for those that dabble in many subjects (assuming you haven't bought them all already). Searchable complete texts can be quite useful.

Safari is also a good way to get access to out-of-print O'Reilly titles.