Is there an alternative to Adobe Flash?

For alternatives as you said, there's Gnash and Lightspark.

To install open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:

For "Official PPA of the Lightspark project":

sudo add-apt-repository ppa:sparkers/ppa

or for "Automated daily builds from ustream git master":

sudo add-apt-repository ppa:sparkers/daily

Installation:

sudo apt-get update
sudo apt-get install lightspark browser-plugin-lightspark

Click here for instructions on using PPAs.

NOTE - Update for Ubuntu 15.04, using Lightspark Version 0.7.2-6 (The above PPA repository does not appear to be working in Ubuntu 15.04)

Lightspark 0.7.2-6 deb package can be installed directly via the software centre, which is invoked by clicking the "Install Using Apt" button on the following web page:

http://www.ubuntuupdates.org/package/core/vivid/universe/base/browser-plugin-lightspark

Depending on what you use Flash for. If you only need it for Youtube and a few well known sites you can use the addon FlashVideoReplacer for Firefox, or the Greasemonkey script Linterna Mágica. LM works on many sites and also on Chrome and Opera in additional to Firefox, though I notice that if Flash is installed it doesn't work on Firefox (Flash player will load while LM is loading, so you have two instances of the video). Also LM doesn't work on Youtube if you use Chrome (it used to, it just stopped working a few days ago)

For Youtube you also have other options like Minitube or SMPlayer (install from rvm's PPA, the new versions have a Youtube viewer)

I have played with Gnash and Lightspark on different distros and different machines. My conclusion is that they are complete waste of time. They never work except for a few demos. I see people recommending them on many Linux sites, either they have better experience somehow or these guys have never tried it themselves.