Finale and Sibelius are, by both price and profile, the leading commercial software packages dedicated to music notation. These are the professionals. Music data is entered through a variety of different means, from manual and step entry, to musical instrument digital interface instruments, scanning and even from microphone input. MIDI files can be loaded and charted automatically. Transpositions and revisions are easy; the user interfaces are intuitive. Performance comes at a cost. Retail price in 2011 for both packages is around $600.
The tasks required of notation software for a composer of symphonies will be more extreme than those of a singer-songwriter who plays piano. Some makers of notation software offer "lite" versions of their retail products for both trial use and for those who don't need the power that the high-priced name brands offer. Finale offers Notepad for under $10. Forte notation software offers a free version geared toward children and a light version at less than one-half the cost of its lead product.
Music notation is well-served by completely free software such as Musescore, NCH Crescendo and Musette. Free programs tend to have fewer features, fewer output options and their user interfaces are not developed with big research budgets behind them. At no cost, however, you have a chance to try several with the goal of finding one that suits your work method.
Many of the notation packages mentioned have rudimentary arrangement, sequencing and mixing capabilities. Digital audio workstations like Pro Tools, SONAR and Logic incorporate notation capabilities as part of their packages. Notation is created instantly by choosing a notation view of an existing MIDI track. Depending on the software, multiple tracks can be combined. Some formatting and editing are possible, although the options are more limited than they are with dedicated notation software.