Akerbeltz has compiled a list of the most common words used in written Gaelic

The online version of Dwelly's great Scots-Gaelic/English dictionary

A comprehensive Scottish Gaelic / English Bidirectional Dictionary incorporating Dwelly. The Faclair nan Gnàthasan-cainnte which used to be on the Akerbeltz site has been integrated into the Faclair Beag.

You can use Google Book's search function to search the dictionary in both directions. Not all pages are available for display, but if you own the dictionary you can use the page number to check in your copy when searching for English words.

Dictionary for Local Government - Scottish Gaelic and English

The dictionary of Sàbhal Mòr Ostaig. The search interface also ties in with a list of other online dictionaries. As with most online-dictionaries, you get a big list of words without explanations, so finding the right one can be a bit difficult sometimes. It will do for starters though, and I use it all the time. It's also very convenient for finding words fast that you have forgotten, or if you wish to read a text and don't want to spend ages trawling though a book for the words. It has the base forms of words listed, so if you run across an irregular form you don't know the base form for, you won't find it. Select Briathrachan Beag for that, it has some inflected forms listed. It's only a little dictionary though, as the name says (beag = little).