Jalview is a multiple alignment editor written in Java. It is used widely in a variety of web pages (e.g. the EBI Clustalw server and the Pfam protein domain database) but is available as a general purpose alignment editor.
http://www.jalview.org
CINEMA is a Colour INteractive Editor for Multiple Alignments.
http://utopia.cs.manchester.ac.uk/cinema
Multiple alignment of nucleic acid and protein sequences
http://www.clustal.org/