Here's a selection of 94 patterns from the group Show your Tartans created back in October of 2007. To see more great plaids and tartans check out these other groups as well: Plaid Patterns, Plaid Pride, Plaids and Tartan Lovers and Wearable or Stareable Plaids. Feel free to link to other similar groups in the comments if there are others I missed.
For those of you a bit confused on the difference between plaid and tartan the short answer is that plaid is a piece of clothing and tartan is "any of numerous textile patterns consisting of stripes of varying widths and colors crossed at right angles against a solid background." To learn more check out this forum and these pages: Tartan & Plaid.