The Danger Hiptop (stylized hiptop), also branded and sold as T-Mobile Sidekick, is a mobile smartphone and communicator series that was produced by Danger, Inc. from 2002 to 2010, developed in close partnership with T-Mobile US and with most models manufactured by Sharp Corporation. The Hiptops were designed to be held horizontally with both hands, allowing typing with two thumbs on a QWERTY keyboard that is revealed by a 'flip out' display rotating 180 degrees on a hinge pin (with the exception of one model, Hiptop Slide). They ran on a cloud-based, Java-made software synchronizing with back-end services provided and maintained by Danger, holding personal data and offering services such as email, instant messaging, and a catalog of downloadable apps.

Danger, Inc. was cofounded by Andy Rubin, who would become the author of Android. The original Hiptop was released on October 1, 2002, manufactured by Flextronics. Described at the time as a "PDA phone", it was notable for combining a cell phone with a full HTML supporting web browser, integrated AOL Instant Messenger, a keyboard and a navigation scroll wheel, running on the GSM and GPRS cell network of T-Mobile. The second generation Hiptop/Sidekick debuted in 2004, manufactured by Sharp, with a slimmer design and the addition of a VGA camera. This was followed by the third generation in 2006 which replaced the scroll wheel with a trackball and added a music player, Bluetooth, and faster EDGE data. The line diversified in 2007 with the Sidekick iD, billed as a budget-friendly model with cut features, as well as the Sidekick Slide, a model manufactured by Motorola and with a slide design instead of a swiveling screen. The fourth generation top model was the Sidekick LX, increasing the display size to 3.0 inches, and this was followed by the Sidekick 2008 and the final model, Sidekick LX 2009, with numerous hardware upgrades including 3G connectivity and social networking software integration—this one was later also sold as the Mobiflip or the Sharp Jump, with modified software and not using Danger's data services.

The Hiptop/Sidekick became popular especially among young consumers, unlike similar devices during its early years, namely Palm and BlackBerry, which found more business success. Danger, Inc. was purchased by Microsoft for $500 million in 2008, who used the technology to build the ill-fated Microsoft Kin. Danger's cloud services were shut down on May 31, 2011, ending the Hiptop/Sidekick data services after almost nine years in operation. The Sidekick was revived later that year in the form of a 4G device running Android 2.2 Froyo, manufactured by Samsung, using a similar form factor to the originals. The Sidekick became incredibly popular in the U.S., at its peak receiving much attention in American pop culture and now considered to be iconic. Some models were also marketed, to lesser success, in Canada (by Fido/Rogers), Australia (by Telstra), Singapore and a number of European countries.

The Sidekick included a backlit monochrome display; the screen could be swiveled to cover the QWERTY keyboard. It also featured a clickable scroll wheel and blinking lights.

The original Hiptop was made official on September 6, 2001, and was released on October 1, 2002, sold by T-Mobile in the United States, initial price was $200 and then $40 per month. A later revision of the Hiptop upgraded its screen from a monochrome LCD to a 65,000 color LCD. This version was released on June 6, 2003 by T-Mobile for a price of $299.99.

From the beginning, Hiptops have featured "Menu", "Back", "Jump" and other keys accessible even when the unit was closed. The Hiptop also featured a speaker which is used for device sounds but not telephone. Along the top edge of the phone bezel is an infrared transceiver. Although Danger stated that the infrared port would come to use in the future for data transmission (i.e. IrDA), this never occurred and the port remained without purpose, and was not included at all in future Sidekicks. The headset jack serves a dual purpose, as it is also used for an optional accessory camera.

SunCom and Edge Wireless used to sell the device in some parts of the United States. Fido was the exclusive Hiptop carrier in Canada and released it in August 2003. In Germany, the Hiptop service was also offered from 2003 by E-Plus which included a flat rate tariff. The Hiptop was also sold in Singapore by Starhub in 2004.

Announced on August 9, 2004, and released on September 22, 2004 by T-Mobile US, the Hiptop 2 (stylized hiptop²) was the first to be manufactured by Sharp. Compared to the first generation, the new model moved the D-Pad (directional-pad), previously found on the inside, to the left hand side of the device, while shoulder dials, volume and power buttons were on the outside edges. The Hiptop2 also features an integrated VGA camera and a speakerphone at the rear of the unit. Also was the addition of page-up/page-down buttons on the top and bottom of its scroll wheel.