WheelTug is an in-wheel electric taxi system under development by a company of the same name, Wheeltug is a subsidiary of and majority owned by Borealis Exploration. The system will enable airplanes to taxi forward and backward without needing a tow tractor or using main jet engines. WheelTug will accomplish this goal through twin electric motors installed in the nose wheels; these motors will be powered by the aircraft's Auxiliary Power Unit (APU). According to Aviation Week, the WheelTug system was projected to provide savings in ground turnaround time and increased aircraft utilization. 
In June 2005 Chorus Motors ground tested the WheelTug concept on an Air Canada 767 at the Evergreen Air Center at Pinal Air Park in Marana, Arizona with an electric motor attached to the nose wheel for taxi testing, the first demonstration of electric taxiing onboard an actual aircraft. Delta Airlines issued a press release in 2007 that Delta would become a development partner and launch customer for Wheeltug expecting installation of first production units on Delta's 737s by late 2009. According to a Wheeltug press release, roller tests were conducted at Prague Airport in November 2010 in snowy and icy conditions, and the first fully 'in-wheel' demonstration unit was tested there June 2012. In December 2016, the FAA accepted the company's Supplemental Type Certification (STC) plan for the Boeing 737 Next Generation models. As of January 2017 more than 20 commercial airlines accepted optional production slots. The WheelTug system is hoped to enter service for the 737NG once the certification process is complete in late 2018, with Canadian carrier Air Transat as the launch customer. 
TaxiBot, a semi-robotic towbar-less tractor which meets and connects to aircraft, currently it is the only alternative E-Taxiing system certified and currently in use. TaxiBot can tow aircraft from the gate to the takeoff point.
Another competitor previously under development by EGTS International, a joint venture between Honeywell and Safran, sought to install ground taxi motors in the main landing gear wheels,  the partnership was dissolved due to the new economics imposed by the sharp drop in the price of jet fuel but Safran continues to work on the concept. 
- Wall Street Journal article by Scott McCartney dated December 29, 2013, "When an Airplane Needs a Taxi;" https://www.wsj.com/articles/when-an-airplane-needs-a-taxi-1387917073?tesla=y
- Aviation Week article dated January 10, 2017; "WheelTug Begins E-Taxi Certification and Promises Pushback Savings" by Graham Warwick; http://aviationweek.com/technology/wheeltug-begins-e-taxi-certification-and-promises-pushback-savings
- Airports International online article dated June 21, 2010;"WheelTug;" http://www.airportsinternational.com/2010/06/wheeltug/5420
- M.I. MD Ithnan et al; "Aircraft Taxiing Strategy Optimization;" Ph.D. research paper, Technology University of Delft, Department of Maritime and Transport Technology, p.6; http://rstrail.nl/new/wp-content/uploads/2015/02/izzudin_ithnan.pdf
- "Boeing Demonstrates New Technology for Moving Airplanes on the Ground - Aug 1, 2005" (Press release). Boeing.mediaroom.com. 2005-08-01. Retrieved 2013-10-23.
- Air Transport World online article dated January 10, 2017; "WheelTug Touts Taxi System's Time Savings;" http://atwonline.com/eco-aviation/wheeltug-touts-taxi-system-s-time-savings
- CBC News online article dated January 18, 2017;"Air Transat to test taxiing system that could cut emissions;" http://www.cbc.ca/news/business/air-transat-wheeltug-1.3941862
- AIN Online article dated February 11, 2014;"WheelTug, Safran-Honeywell and IAI Offer Three Rival Solutions for Airline Engine-off Taxiing;" http://www.ainonline.com/aviation-news/air-transport/2014-02-11/wheeltug-safran-honeywell-and-iai-offer-three-rival-solutions-airline-engine-taxiing
- flightglobal.com online article dated 24 February 2015;"ANALYSIS: IAI to start widebody TaxiBot certification in autumn;" https://www.flightglobal.com/news/articles/analysis-iai-to-start-widebody-taxibot-certificati-409267/
- flightglobal.com article dated 13 July, 2016;"FARNBOROUGH: Honeywell and Safran halt electric taxi project;" https://www.flightglobal.com/news/articles/farnborough-honeywell-and-safran-halt-electric-tax-427400/
- aerospacemanufacturinganddesign.com article dated June 22, 2017;"Airbus Paris Air Show 2017 highlights;" http://www.aerospacemanufacturinganddesign.com/article/airbus-paris-air-show-2017-highlights-062217/