Correct, the Wilson Cellular Repeaters are among the best out there, but like the "passive" method above, it requires installation of an antenna on the roof. The Repeater modules themselves have their own short "stubby" cellular antenna on them or built inside for mounting interior to the home. Also, the OP did say he was looking for a "free" solution, so I suggested the above. There are also ways to craft your own antenna, such as from a Pringles can or other container, to make a "cantenna". If you search on Youtube for them, there are plenty of DIY instruction videos.
There are better home solutions as you mentioned. OP mentioned free. The better home solutions can carry Upto 5 different devices at one time. They also allow roaming throughout a single room. They are also far from cheap. I didn't mention the yagi due to cost. For whole house solution a femtocell would be best. Cheapest room/multi device from Wilson is about $300. The sleek was closest to free. It can also be powered by battery pack for emergency while hiking.
Agreed, and as both I and you said, the OP was looking for a free solution. I also agree that there are both less and more expensive solutions. I believe we are both saying essentially the same thing.
Back to what the OP inquired about.
Here (
My homemade projects: DIY cell phone GSM 3G signal booster antenna), is a home-brew booster which uses common household items (even the coax cable can be either the standard RG-8 or you can use common cable TV cable, RG-58), to manufacture. I found this with a Google search for "DIY Cellular booster". It doesn't take a whole lot of talent. Just some basic know-how for measuring the length of the wire, some basic geometry to understand and reproduce a 90 degree bend, and some low-level mechanical skill with pliers, tape, scissors, a wire cutter, a hacksaw or Dremel with a cutting wheel, and a screw driver. The terminal blocks are very inexpensive and can be bought at your local Radio Shack or other electrical supply store.
The pics of his phone with 2 bars before and 5 bars after at the bottom of his blog are pretty impressive.
Truth is, a simple piece of coax that is stripped back to expose the center conductor, made at a length that is close to half the length of the wave can be an effective antenna. In Cellular, there are several bands that are used, but the most common for CDMA phones (think Verizon), are 800Mhz and 1,900Mhz, and the 1/4 wavelength for these two bands is 3.51" for 800Mhz and 1.48" for 1,900Mhz. So, if you took the end of a coaxial cable and stripped it back, exposing the center conductor that is 3.51" long, and then twisted the shield into an opposite 3.51" conductor (3.51"-3.54" is a reasonably acceptable length variance since at 3.54" it would cover a range of frequencies near 800Mhz and below), and placed them at diametrically opposing orientation (180 degrees), you've effectively created a standard 1/2 wave dipole antenna (see below), which measures 7.02" tip to tip and that alone is sufficient for the external antenna to capture the incoming signal and transmit the outgoing signal. For the 1,900Mhz band, the antenna's two ends would be 1.48" instead.
For the internal end, you could actually simply do the same thing again. The amount of "Gain" you would accomplish is minimal, but it may be enough to get you over the hump. Furthermore if you hold the inside end's center conductor right against the phone's back as shown here:

...the minimal gain will be enhanced due to the phone's close proximity to the primary element conductor of the cable carrying the signal out and in. In the DIY antenna above, he's managed to make an external antenna that had considerable gain, probably in the range of 7-11dbm, and that helps to effectively amplify the signal at the antenna on the phone, since the "collector" antenna is a larger element and is balanced, tuned, and likely low in SWR. It also helps tremendously that the external antenna is attached high up on an antenna pole.
All that said, inexpensive or free solutions are rarely the best.