Taxonomy Single Tag improves core's taxonomy autocomplete by allowing commas to be used as part of the taxonomy term. To do this, TST limits autocomplete taxonomy fields to a single term.
Why such a module might be necessary is explained at Taxonomy autocomplete without multiple select:
Is it possible to create a taxonomy-input with autocomplete, but without the possibility to select multiple terms? [With] "Free tagging" it's possible to separate multiple terms with a comma, but I [would] like to select only one term. Autocomplete is necessary because 1000+ terms are too much for a single dropdown...
Drupal 7 version
There is now a Drupal 7 port of this module. It takes a somewhat different approach than the D6 version -- Taxonomy Single Tag exposes an "Single tag autocomplete text field" widget. This widget can be set to single- or multiple-entry, allowing unlimited tagging with commas and quotes.
When adding a field of type "Term reference," select the "Single tag autocomplete text field" widget option to force the autocomplete field to accept a single term and allow commas. As with the D6 version, this field can be used to add new taxonomy terms or select from existing terms.