Pipe bombs are a standard of improvised weaponry, probably because they are relatively simple to construct. Explosive material, a fuse, and a casing make for a deadly if unpredictable weapon. Opposition fighters like Syria's Tenseekiet Union have online videos showing off their bombs, some of which are made using the explosive material from un-exploded bombs. With access to better weapons, expect pipe bombs to fall out of favor as soon as rebels can reliably get grenades.
A major limitation of grenades and pipe bombs is that humans just can't throw them very far. Enter this slingshot. In the video found here, a two-man team places pipe bombs into the pocket of the slingshot: one man aims and pulls the slingshot, the other lights the fuse with a disposable Bic, and boom! The bombs fly over a wall.
Dubbed the Sham II, this armored fighting vehicle is very much a poor man's tank. Built out of a re-purposed chassis and inch-thick steel plating, the Sham II has room for a driver and a gunner inside. Cameras on the vehicle give the driver a nice four-screen view of the area around the tank, and there's a camera mounted on the turret so the gunner can see where he's firing. The turret is controlled by a converted PlayStation controller, and the gun appears to be a 7.62 machine gun, firing the same ammunition as an AK-47. With the embargo lifted, tank sales to Syrian rebels are a remote possibility. Germany is fine exporting tanks, having sold 100 to Indonesia earlier this month, but getting tanks and spare parts into rebel-held areas is likely a difficult task.
The Hell Cannon
The Hell Cannon is a massive piece of artillery, capable of launching explosives about one mile. It's a major production, can mount two rockets on the barrel, and it even comes with a fact sheet! Syria watcher Brown Moses collected several videos about the Hell Cannon. To me it looks like the weapon is as much a propaganda push as it is a functional tool of war. The main projectile used by the hell cannon is a re-purposed propane gas cylinder, since ammunition for improvised weapons is also generally hard to find.
This device is a modified shotgun, made for firing pipe bombs farther than they can be thrown. With a cup on the end of the barrel to hold the pipe bomb in place, a modified shotgun cartridge is fired to launch the bomb. A second person is still needed to light the pipe bomb as it sits in the barrel. With the embargo lifted, it's likely Syrian fighters will instead pick up Croation-made RBG-6 grenade launchers, already found in Jordan.
Found on a Youtube playlist of Syrian DIY catapults, this one most resembles the medieval rock lobbers of yore. Catapults remain remarkably efficient weapons, if large and difficult to move into position, because there is no fuel cost involved. Weights, tension, and gravity combine to great a powerful downswing that hurls a projectile skyward, cruising above rooftops before crashing on a target below. Efficient as it is, a catapult has nothing like the accuracy of modern artillery. Given the chance, Syrian rebels would gladly adopt these European howitzers and smart rounds instead.
A Partial Tank
This converted turret has found new life as makeshift cannon, resting on a trailer pulled by a truck. Scavenged parts of war machines are, in the absence of new weapons or vehicles, the next-best way for rebels to arm themselves.
Indirect fire is a really useful way to attack--you launch explosives that soar past walls and buildings. Because indirect fire is so popular, however, there's a reasonable chance that launching a mortar attack on an outpost means risking mortar fire back. There's a whole science to this, but none of that matters if the attacker can fire shots and then move away before the return fire lands. This truck-mounted mortar system does exactly that.
Machine Gun Robot
It's not exactly a remote-controlled vehicle, but this corded machine gun is the very distant cousin of advanced armed combat robots like the Talon. You steer it by manipulating a controller attached to the robot's leash. The gunbot doesn't have the tracks and treads that enable other vehicles to smoothly navigate rough ground, and it can get stuck on as little as a rock. Still, it's much better to send a robot around a hostile corner than a person, and a robot that can fire back is even better.
A Wireless Machine Gun
This is a Soviet machine gun mounted on a stand and fired remotely. Great, provided it can be set up before fighting breaks out and the person controlling the remote has some way of seeing where the gun is aimed. This short video suggests that the gun doesn't have an attached camera. Also, it looks like the machine gun can't fire at full speed, as the recoil from a single shot almost knocks it over. There are better remote-controlled guns out there. South Korea has several incredibly high-tech ones watching the DMZ, but it's unlikely the Syrian rebels will need those. Of course, if the war drags onto a 60-year stalemate, they might reconsider.