So, addon development has been pretty active since early on in the beta, and there are some great ones out there.

The ones I recommend as a base must-have are these:

• Hey!Listen! allows you to get audio-visual cues when your HKM (or other charge-ups) are available for use. It's not currently supported, so some of the things might not work. I tested it and the HKM audio cue works just fine.
• Airii gives you overlay information on various flight departures/arrivals for drop ships.
• Personal DPS Meter helps you keep track of how well you are doing.

Honorable mentions:

• Boing! allows you to hop between various shards to maximize dynamic ARES mission rewards (melding tornadoes, incursions, etc.)

To see more tasty addons, check out Curse and the Firefall Forums.