Jaish al-Islam is arguably the strongest of the seven rebel groups fighting under the Islamic Front umbrella. Primarily fighting in Damascus, it takes the brunt of the Pro-Assadist forces, including fighters of Hizbullah and the Republican Guard.
Formely fighting as the Platoon of Islam and Liwa al-Islam, the brigade restructed itself into Jaish al-Islam. Once part of the Syrian Islamic Liberation Front, Jaish al-Islam later became one of the founding members of the Islamic Front, under which it still fights today.
After being arrested by the Intelligence Services in 2009, Zahran Alloush was imprisoned and charged for practising Da'wa, for which served two years in prison. Zahran Alloush was released from Sednaya Military Prison in June 2011 as part of a government amnesty. Yet, it is most likely prisoners like Zahran Alloush were released in order to further strengthen the process of islamification of the opposition.
When still fighting under the name of Liwa al-Islam, the brigade began to receive financial support via private channels from Saudi Arabia, enabling Liwa al-Islam to grow significantly.
The flow of money was apparently enough to allow Jaish al-Islam to work on two L-39s in an effort to establish a local air force. Based at Kshesh, this project was abandoned after the capture of the airbase by the Islamic State. A video showing the L-39s can be viewed here.
Jaish al-Islam also used the money for a wide amount of other projects. Notably was the usage of a captured 9K33 missile system to shoot down at least two Syrian Arab Air Force (SyAAF) helicopters  and the purchase of two T-72AVs and two BMP-1s from a corrupt officer in the 4th Armoured Division, an elite unit within the Syrian Arab Army.
here and here.
Most interestingly however was the foundation of a local arms industry, capable of both manufacturing a wide array of munitions, performing overhauls and upgrades on captured fighting vehicles and producing small arms.
Below a list of Jaish al-Islam's most ambitious arms projects.
The 'Dushka' bolt-action sniper rifle, using 12.7×108mm rounds from the DShK heavy machine gun.
A 23mm anti-materiel rifle, the largest of its kind currently used in the world. The rifle can be seen used here.
The integration of 107mm Type-63 multiple rocket launchers (MRLs) on a new chassis, improving its off-road capability. This system can be seen in action here.
The introduction of the Arrow of Islam 3 MRL, armed with three 122mm rockets installed on a 6x6 Ural-375D. The Arrow of Islam 3 was mainly used against Nasriyeh airbase, home to a large portion of the SyAAF's fighter-bomber fleet.
However, Jaish al-Islam's main focus is refurbishing and upgrading various types of armoured fighting vehicles. All overhauls and upgrades are believed to be carried out by just a single workshop in Eastern Ghouta.
A previously unarmed BTR-60PU-12 command vehicle, once captured along with the 9K33 Osa and now adapted for use on the battlefield, upgraded with add-on armour and armed with a 23mm gun taken from a ZSU-23. A Czechoslovak made AMB-S armoured ambulance vehicle was upgraded in a similar way.
A BREM-2 armoured recovery vehicle (ARV) captured at Batallion 559, an army Storage Base North of Seen airbase. While not of much use for Jaish al-Islam in its original role, the vehicle was not let go to waste and was reconfigurated into a fire-support platform armed with a 14.5mm ZPU-4.
Jaish al-Islam's tank fleet consists of a few T-55s, multiple T-72M1s, one T-72M1 equipped with the Italian TURMS-T fire-control system (FCS) and numerous T-72AVs. At least one T-72AV was upgraded with additional armour on the rear and on its glacis plate.
More interesting is the upgrade of at least two T-55AMs with homemade armour mounted on the turret and front of the vehicle. It is unknown if the armour contains any explosives and metal plates, also known as ERA. One of the two vehicles can be seen in action here.