As it says in the UF FAQ:

Q. Can I put a favourite UF comic strip on my own website?

A. Yes -- you can assume this FAQ gives you express permission to do so with the following qualifications:

1. You aren't earning money from placement of the strip (e.g. putting it on a page with ads);
2. The strip may only be placed on a page that has NO adult-oriented material;
3. The copyright notice and attributions are left intact;
4. The strip is clickable and links back to http://www.userfriendly.org/
5. The strip itself is not modified in any way.
6. You understand that the permission granted is revokable at any time.

You can do this for up to six (6) User Friendly strips. Beyond that, you'll need personal permission in form of a reprints contract. Note that this might involve a license fee.

I reccomend reading both the site FAQ linked above and the Comment Board FAQ linked to on the left.