Several months back, I was pleased to find the USGS's high resolution orthophotography; it's public domain, so it can be used in OSM, and almost* the entire state of Maryland is available in 6" (15 cm) resolution. As I was touching up some power lines near where I work, I realized that the resolution was high enough that I could follow individual lines, which let me sort out some things that I couldn't see well enough in the Yahoo! imagery and couldn't get close enough to the lines to see in person.
I decided to set a goal of mapping all of the power lines in the state (more precisely, to edge of the state's aerial photography, which extends a small amount into neighboring states), including cables, wires, and voltages. (The voltages came from some state documents available online.)
As of this evening, I'm done. Every major power line (>100kV) in the 2007 aerial photography is mapped, and every power line from TIGER is either verified to exist, retagged as a minor_line, or deleted (if nothing actually exists where TIGER had the lines).
I think my next project will be the railroads in Maryland. TIGER's not bad with them, but I've been adding all the tracks when there's more than one and giving them appropriate names. (TIGER mostly uses the railroad operators, and often past operators, in the name fields; I've been putting those into the operator tag and putting the lines' actual names into the name tags.)
*The lower half of Harford County is missing. I can only assume this is an oversight, since the entire rest of the state, including the sparsely-populated Garrett and Alleghany Counties, is present, plus the layer for the northern half of the county is named "MD_HarfordCounty_0.5ft_Color_Feb_2008_01", but there's no "..._02" layer.