From the Hardware-HOWTO I know there is Trimble Mobile GPS available for Linux. You may also connect a GPS via a serial port. Most GPS receivers have a data port and can connect to a PC with a special serial cable.
Differential GPS is a technique to apply a correction factor from a known location to a GPS signal. This can substantially reduce the uncertainity in the GPS location. Normally the correction signal is acquired using a special radio receiver: dgpsip allows you to receive a DGPS signal via TCP/IP, and send it to the GPS connected to your serial port.
DGPS is a project to put together a low cost hardware and software solution for Differential GPS (in both real time mode using RTCM correction format and in post processed mode).
gpsd is a daemon that listens to a GPS or Loran receiver and translates the positional data to simplified format that can be more easily used by other programs, like chart plotters. The package comes with a sample client that plots the location of the currently visible GPS satellites (if available) and a speedometer. Added support for the DeLame EarthMate as well as a new 'speedometer' mini client.
The QtGPS package contains a piece of software for UNIX/Linux/X and a GPS receiver. It performs logging and replaying of a journey, supporting a moving-map display. QtGPS works with Lat/Long and British OSGB (Ornance Survey) co-ordinate systems.
GRASS (Geographic Resources Analysis Support System) is a free software raster and vector based GIS, image processing system, graphics production system, and spatial modeling system.
XASTIR is a free APRS (Automatic Position Reporting System) program. APRS(tm) was developed to track mobile GPS stations with two-way radio to convey position reports, messaging, weather and more. XASTIR plots this information on a map on your screen where you can see the entire world or zoom down to street level.
as-gps contains a basic support library for accessing the inexpensive ($20) Aisin-Seiki GPS Module previously available at mavin.com. The package also includes several simple console utilities for dumping satellite status, location, and time and for synchronizing the system clock.
gmap is a map viewer with emphasis on temporal data. It hopes to evolve into a free and powerful Geographical Information System.
gps3d is a set of utilities that lets you manipulate your GPS from your Linux box. One nice feature is the ability to view GPS data (track, waypoints, fix, etc.) on an OpenGL, 3D texture-mapped model of earth.