(This post has been updated with information from the comments)
Sure, a brand new section of trail along the Anacostia just opened on Monday, and yeah, it connects two major trail systems and more than 70 miles of trail overcoming a jumble of barriers like highways, railroads, streams and such and it's amazing and you've wanted it for a long time, but now you're bored. I mean Monday was last month. What has happened lately? Well don't worry, there are lots of other new trails that you can longingly gaze at over the shoulder of the Anacostia River Trail while it drones on and on about that thing Nancy from accounting said that was just so unbelievable.
Here is a list that is likely both incomplete and inaccurate (the old Washcycle doubleshot).
The Klingle Valley Trail - DDOT is currently building a 0.7 mile long trail on an abandoned section of road in the Klingle Valley (War on Cars!)
Rock Creek Park Trail - The trail is, right now, undergoing the first part of a long desired rehabilitation which will also add a mile of trail along Piney Branch Parkway
Needwood Rail Trail - Phase 1 of the this trail extending from the ICC trail is under construction.
Bowie Heritage Trail - work was underway in April
Four Mile Run Trail - Work begins next week on the restoration of the lower section of Four Mile Run stream. "As part of the construction, the Arlington trail will be completely rebuilt to current standards, including a new sub-base and asphalt surface. Improved trail railings and a new observation platform will also be installed."
Washington Boulevard Trail - Work could begin in 2017
Capital Crescent Trail - This popular trail will be extended from Bethesda to Silver Spring if/when the Purple Line gets built
Oxon Run Trail - This was issued permits over a year ago. So that's something, right? Also there is an unfunded section in PG County from Southern to Naylor that remains on that county's priority list
The Old Cameron Run Trail - Design could begin in 2017
Northwest Branch Trail extension - from Wheaton Regional Park to Matthew Henson Trail, this is Montgomery County's top hard surface trail priority. Other trails include the Wheaton Regional Park Through Trail, Muddy Branch Trail, Magruder Branch Trail extension, and the Rock Creek Trail - Millennium Trail Link
I-66 Trail - More likely than not?
U.S. Route 110 Trail - Still in phase 2 of 5 planning phases. Same as in January 2013.
Shepherd Branch Trail - Soon to be acquired!
The New York Avenue Trail - A concept
Arlington Boulevard Trail - An existing trail that could be improved
Palisades Trolley Trail - A thing
Other PG County Trails, including a list of high priority, but unfunded trails that has been mostly unchanged over the last 5 years (the Bowie Heritage Trail is off the list and the Seat Pleasant section of the Chesapeake Rail Trail and the north extension of the Rhode Island Avenue Trail are on). This includes the WB&A Trail to Anne Arundel County, Piscataway Creek Trail, Henson Creek Trail Extension, Little Paint Branch Trail Extension, Prince George's Connector Trail and the Folly Branch Trail.