Here are a few useful links for Scottish Gaelic:

Sabhal Mòr Ostaig - Gàidhlig air an Lìon: http://www.smo.uhi.ac.uk/gaidhlig/english.html
This is probably the single best site for Gàidhlig on the web. Sabhal Mòr Ostaig is a Gaelic-speaking college on the Isle of Skye and the site has a lot of links to useful info and learning materials.

Pronunciation and Grammar guide at akerbeltz:
http://www.akerbeltz.org/fuaimean/fuaimean.htm

Dictionary - Am Faclair Beag: http://www.faclair.com
Many of the online dictionaries have display problems with the accented letters, which can be hard for learners to figure out, but this is a basic one that works.

mygaelic.com: http://mygaelic.com
Especially good site for children.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

There is a Fóram na Gáidhlig as well, I believe. http://www.foramnagaidhlig.net/

An Ideal place to find Speakers of Gaeilge na hAlban