Linux PPTX presentation Viewer
Other than LibreOffice, you could use an online conversion tool like Google docs to convert presentations from .pptx either directly or via a Firefox extension. There are other online conversion tools like Zamzar to convert the .pptx to a format that you can access on your machine; format choices include:
.html.pdf.odp.png
Stage (the presentations component) of Calligra is able to open .pptx files.
Kingsoft Office 2013
This is sort of the Chinese version of Microsoft Office, but with support for English and a few other languages, and has be simply brilliant in the latest versions. It has a nice interface compared to the bog standard libreoffice - and versions later than the 4207 2013 version can edit Office 2010 .docx documents, as well as pptx and Excel formats.
It is available as a one-install package, which provides versions of Word, Presentation/Powerpoint, and Calc/Excel. For most uses, Libreoffice will workgreat, but this is sometimes necessary for compatibility with unimplemented pieces of M$ Office formatting.
I will also note that any North Americans who may want to use this may have a problem, due to licensing things - https://askubuntu.com/a/235960/178596 EDIT: this may not the case anymore
As of 2013: Be sure to read their restrictions: AMERICANS downloading it are violating their license and confirm they may be sued. http://wps.cn/privacy/unixdistribute "Kingsoft authorizes you the non-exclusive right to use the "Product" in the territorial scope in the world (“Authorized Territorial Scope”) except for North America (including only the United States, Canada and Greenland, similarly hereinafter)" Credit to @Rinzwind
You can download the installation DEB or RPM files from here: http://wps-community.org/downloads
There is a guide to installing .deb files here: http://www.cyberciti.biz/faq/ubuntu-linux-how-do-i-install-deb-packages/
For rpm's, you can cd to the directory where it has been downloaded, then sudo yum install PACKAGE-NAME.rpm for Fedora and other rpm systems.
For both this AND libreoffice, you may get better compatibility if you can get the Windows fonts and install them onto your Ubuntu computer. You can copy them from the fonts directory on a Windows computer on its C: drive.
When using it you can change the interface etc from toolbar style to ribbon style. Also may I suggest saving in a format other than the default (e.g. docx or odt if possible), it makes it much easier if you don't have installed elsewhere as its format not supported that widely.
Please note this usually can be found as FREEWARE, not freely or open source. You will almost definitely want to consult the Terms & Conditions, and Privacy Policy (possibly this one) before installing.