Steve Park Horn Music offers a number of mp3 downloads of piano accompaniments to horn repertoire such as Adagio & Allegrio, Alla Caccia, Beethoven Sonata, Bozza En Foret, and many more.
SoloConcertos.com - offers “MP3 downloads of computer simulated orchestral accompaniments for concertos (without the solo part). We use advanced software to replicate the experience of a real orchestral environment, allowing you to enhance your practice with convincing orchestral accompaniment. Our practice tracks offer a variety of benefits to ensure your performance preparation is both effective and fun. All you’ll need is a set of speakers, an MP3 player, and your instrument of choice!” For horn the site currently offers accompaniment tracks for Haydn 1 and all the Mozart Concertos. You can listen to demos online. Each concerto is available in several tempos (included in the very reasonable price); there are ritardandos and rubatos at the usual places.
Smart Music is the 10 ton gorilla of the software accompaniment department: “More than 30,000 solo and ensemble titles and counting!” For all instruments + vocal. Includes not only multiple versions of all the standard solos and chamber music pieces, but also many exercises, band methods (e.g. Accent on Achievement Bk 1 & 2), as well as the Aebersold jazz accompaniments (!). Lots of transcriptions, including the Mason Jones Solos for Horn and Canadian Brass collections. Besides the stupendous choice of material, what makes Smart Music especially interesting is that it listens; it is capable 1) adjusting to the the tempo of the player and 2) “gives constant feedback [to the player], and sends a recording to the teacher.” Best is to take out an annual subscription; your school probably already has one. If it doesn’t, it should.
GarageBand - Part of Apple's iLife software package (Mac only), GB makes it extremely easy to construct backup tracks of infinite variety. Not especially useful for constructing accompaniment tracks for chord progressions; for that, see below.
Band-in-a-Box - does a good job of providing a tireless piano, bass, and drums (among other possibilities) for improvisation practice over chord progressions.
Auto Accompaniment Software - Hitsquad Musician Network is a vast collection of music shareware of all types. This link takes you to the page of “auto accompaniment” shareware programs, which come in 3 license categories (free, demo, shareware) and 3 operating systems (Mac, Windows, Linux).
WorldJazz - PlayAlong CDs - Pick the 10 jazz standards you want accompaniments for from a list of 330, and these folks will put it on CD for you. 5 versions of every song available. 4 levels of each category: jazz standards, classic latin, latin jazz, blues, funky pop, fusion jazz, rock, and more.