Here are some useful links I have found whilst trawling the web. Most are kilt or at least Celtic related, while others are not. More sites will be added to this list over time. If you find an interesting link, feel free to send it to me.
Scottish Tartans Authority A charity devoted to the study, recording, preservation, and promotion of all things tartan. Many of the website's features are free while others require a paid yearly membership.
The Scottish Register of Tartans A newly founded government institution which features a searchable tartan database. Membership is free.