Here are some of the main suppliers of useful modern-urban downloadable models that I have found so far, and a couple do some freebies.
Basic Urban Terrain
Fat Dragon Games - under the 'Free Stuff' tab are a few download links. Of particular note amongst these is the January 2008: Gridless streets and rooftops for E-Z Heroes, which includes a straight, t-junction and crossroads with road markings. Fat Dragon also do a pack called E-Z Heroes for $9.99; the description reads:
EZ-HEROES lets you convert any box into a 3D map for your click base hero miniature game. You get full color 300dpi pages of various brick and wall styles, numerous windows and doors, automobiles, business signs, roof access structures, fire barriers, smoke barriers and plenty of 1.5" gridded ground textures like concrete and streets to make your city. Set includes over twenty five pages of models and a full color instruction manual.
Microtactix - offer 3 free sample buildings, which can be found by looking through their products on offer via the catalogue links (the buildings are a deli, pizza shop and TV and Radio store); one of these (TV shop) can easily be modified from two-storeys to become 3 or more storeys. In addition, the deli features a small roof access and air conditioning unit which can possibly be used to modify the other two buildings. Thus, with the three free buildings a few small footprint buildings can be created with some variation of height. The catalogue is less clear than Fat Dragon's offer, but there are currently 3 packs of buildings for Twilight Street. Fans of more abstracted building may prefer the Dirt Cheep Cityscape that is available. This is the blurb for one of the Twilight Street sets:
Twilight Street Heroic Scale has seven big and beautiful new building kits (eight, counting the free preview set) plus billboards, awnings, water tower, street lights, and three full pages of custom signs! Also included is a complete set of customizable street tiles with the standard 1 1/2 inch square grid used in most click-base games! Now your superheroes can work the dark, grim side of the city with Twilight Street Heroic Scale!
The good news is that with just the free stuff available, a printer and enough materials (paper, and optionally card and/or foam card), it is relatively easy to assemble a small cityscape. If buying a pack, many more options are opned up, and in this the Fat Dragon offer seems better organised and probably better value than Microtactix.
World Works Games - Modern ranges; a selection of sub ranges. There looks to be some very cleverly designed products offering more than just simple boxes. There are also 'kitbashes' devised by product users.
The Virtual Armchair General - Mean Streets range; can be bought ready printed or as PDFs. Very 1920's/1930's era feel to the models. They do some sets of buildings so a good look around the site may be of help.
Canon: craftown - some potentially useful buildings and vehicles here. They may be a little too pristine and clean for some tastes, as well as being very basic, but a nice if small selection of vehicles is on offer for free.
Illinois Historic Preservation Agency: build your own Main Street - some HO scale downloads that could possibly be scaled up in printing to be of suitable scale.
Street Furniture/Small Scenic Pieces
MegaMiniatures - Megaminis offers some pretty inexpensive pdf's via the link, including 5 different vehicles kits, as well as crates, dumpsters and vending machines.
Tommygun - this link leads to Warseer, an independent Games Workshop oriented site. In amongst the various threads is one by Tommygun. This is his blurb:
The purpose of this log is to distribute free printable terrain items for use on terrain boards or on models.
Items like boxes, posters and anything you can use as props.
Have a good look through his stuff, you may be surprised at the sheer quantity of items he has created. It will mean working through a few pages of the thread, but it should be worth it for gamers looking for modern papercraft terrain.
GermsWorld - does some basic 25mm urban building PDFs in a 'shoebox' style (with lift off lids), as well as a street furniture PDF.
Jim's printable Minis - offers a variety of decorative items for interiors (everything from offie supplies to home entertainment, as well as pizza boxes etc), as well as some US road signs.
Toposolitario.com - offers some ISO-style containers (sci-fi influenced), as well as a warning signs pdf.
RavensBlight Toy Shop - a couple of spooky buildings, as well as a car PDF which should be suitable if printed at about 50% size I think.
I have not so far used papercraft terrain, but for various reasons I may soon do so, so in searching for good stuff, and in doing so trying to keep costs as low as possible, the above links are the best of what I found. If anyone has experience with any of these creations, or has links of their own, please comment offering your insights - it may help another gamer who happens by.