We (the Rune using posters over at Deathknight.info) have complied a list of Death Knight AddOns on a stickied thread on our forums. Some of the mods on the list are Death Knight specific, others are more general mods but are useful for Death Knights
DK Mod List:
http://deathknight.info/forum/index.php?topic=3272.0
Also check the DeathKnight.info Runes AddOn that was developed by a forum poster on DKI with a lot of input from the DKI community.
This mod is very flexible. You can change the background graphic, reorient, remove the background, have numerical cooldowns (OminCC support is baked in), have a HP/RP display, etc. The Runes used in the mod are a result of a DKI contest to develop better looking Runes. The mod started as a project to use the Runes in an existing mod and taken off from there based on feedback from the community (include mine
)
http://wow.curse.com/downloads/wow-a.../dkirunes.aspx