I think most of us post links to other sites occasionally. There are some great resources out there. I regularly post links to the profiles section of cichlid-forum and use plantgeek, apc, and planted tank for info on plants. Like Fort said, as long as you're not referring people to other sites (for membership, profit, etc), and it's just to help someone, it's perfectly fine.
Now... if it's a signature, that's a different story:
Signature may not include email addresses, links to competing websites, prompts to contact or a sales pitch
Just thought I'd toss that out there as well while on the topic.