How to accept donations?
Addresses are intended to be used once, and there are problems with reuse, but it is usually safe to just use the same one repeatedly for donations if you don't have a way to update it easily. However, Bitcoin is still a new technology, so this may change in the future (making reuse stop working) - read the release notes of new versions before you upgrade to ensure you're not caught by surprise.

If you have a way to update the webpage automatically, there is a JSON-RPC method "getnewaddress" that you can use. However, it is very unsafe to run your Bitcoin wallet on a webserver, so be sure if you go this route that you push it from a separate, secured machine.

PayPal does not at this time support Bitcoin.