Before investing a lot of money, students should consider their choice carefuly.
While spell checking software is a great tool - especially for editing typing errors - no grammar or translation software can be a substitute for the human brain. Most students with a basic academic knowledge of a language will already have a better knowledge of the language's grammar and can also resort to using some of the excellent, and inexpensive grammar books that are available.
Spell checking and basic grammar correction suggestions come as part of most word processing programmes, and is therefore already free. While machine translations, such as the free one in Google which is one of the best, (they pretty well all use the same engine) may be helpful for grasping the basics of a short text in a foreign language, using them for a target language from your own language can provide nonsense or even have dangerous consequences. The resulting text is often worse than a student's own first draft, and will always
need heavy editing by a native or highly competent bilingual speaker.
Some expensive grammar software might need to be avoided completely: http://www.englishforums.com/English/WhitesmokeComReview/drdvg/post.htm