Luxand has released Echo Effects for After Effects, with five properties to adjust: Echo Intensity, Trace Length, Oscillation, Phase, and Frequency. Any advantage over AE's built-in Echo Effect is not been made explicit.
Update: Topher Welsh has a tutorial on Echo below, inspired by a recent music video.
Maltaannon also recommends another built-in AE filter, CC Time Blend. For background on that filter check out Blend and Bend Time by Eran Stern and the old-timey classic CC Time Blend FX with Brian Maffitt posted recently by Motionworks.