What alternatives to LibreOffice (specially LO Writer) does Ubuntu come with. I have seen Abiword there but are there others?
KOffice Calligra Suite
IBM Lotus Symphony
uses code from OpenOffice.org and includes enhancements such as new sidebars in its user interface.
Not in the repositories, but I think it is worth mentioning, you may give it a try and see what it brings new to OpenOffice.org besides the interface
Other Useful Options
For calculations one could use gnumeric, octave and Python with numpy. A fairly good lightweight word processor, Abiword is available.
For document creation, Latex and Context are excellent systems, the learning curve is a little steep when it comes to Tex based document creation systems, but when one starts reaping the rewards of the power of these systems one will see it is a small price to pay.
Scribus may also be considered, while it is a page layout program and not a word processor there is a story editor that have similar behaviour to a word processor and there are many instance where something like this is better for document creation than a word processor.
For drawing Xfig and Inkscape are quite good.
For database, sqliteman interface to the sqlite database system
There is also WPS Office, a Chinese Microsoft Office clone.
Note: I haven't tried it out myself yet, but you will find a number of testimonies in the comment section of the link above.