I have built my first bundle for TextMate, the famous text editor for Mac OS X. The bundle allows simple DNA sequence analysis tasks, like setting the reading frame, sequence comparison, expressing codons, and determining masses of the expressed amino acids. For sequence comparison, I integrated two tools from EMBOSS: needle and water. These are also accessible via a web interface of the European Bioinformatics Institute.
You can download the bundle here. Just unzip the file and double click it or drag it on the TextMate application. When you edit a sequence file you should select "Fasta" as language at the bottom of TextMate. Then, you can access the sequence analysis commands by pressing Ctrl+Escape.
Update 2008-04-13: Configurable sequence highlighting.