Is there any kind of software or a DIY-hack, that can give one the effect of having a bassist, without actually having one. Of course, I do not expect it to be half as good as a real bassist, but something that gives an additional colour to the music being played. Perhaps some software, which follows one of the other instruments, and plays matching bass notes. Such a thing might exist as a guitar-effect (perhaps a VST?).
Okay - so you don't have a Bass player for your band but want a software based stand in to add a bass line to your music until you find a bass player. Or perhaps you just don't want any more band members.
I can't give you any source of free software that will automatically add bass parts to your arrangements. But I will offer two solutions that will allow you to "have the effect of having a bassist - without actually having one", and will not require you to play a bass line because they automatically create or generate the bass line for you. All you have to know are the chords in your song to use the solutions mentioned below.
First solution you might try is an effects pedal called "Trio Band Creator" by Digitech ($179.99 US). It will automatically add bass AND drums but the bass and drums have independent volume controls so you can have it play only bass whenever your drummer is there. It will automatically add a bass line by listening and analyzing the chords you are playing. You might even be able to use it to generate automatic bass backing tracks by recording the output. Here is a link to You Tube video demonstrating this tool - Digitech Trio Band Creator Demo
The second option that I am familiar with - would be a songwriting program that will automatically put in your bass line based on the chord progression you give it. One that is very inexpensive ($84.99 US) and easy to use - is Band In A Box by PG Music. It's software that allows you to create complete song arrangements by typing in the chords using standard chord symbols like like C, Fm7, Gdim etc. Of course you also indicate how many measures for each chord. For your purposes you would just have it create a bass accompaniment based on your chord progression.
You tell it what style music and set the tempo, type in the chord progression on the timeline - and it does the rest. It will automatically generate the bass accompaniment for you based on the chords you input and the style of music you select. And if you want to add a piano player to your band for some songs, Band in a Box can do that for you as well (or any other musician for that matter). You would then need to use the finished arrangement (consisting of the bass line) to create a backing track which could be stored on an mp3 player such as your i-phone and select and play the backing track through your amp or PA.
Here is a link to Band In A Box - Band In A Box Description
If you have an i-phone, i-pad or i-pod touch, Apple has a Garage Band App that you can download from the app store for less than $5.00 US. Among the features listed are "smart bass". I know you can use it to create midi bass lines and use various style basses, but I'm not sure how automatic it is - I have not used it. Check on line for reviews. You can start here - Review of Garage band app
Good luck and have fun with your band!
I have a piece of software on my android tablet called "Chordbot" that cost about £3. It is intended as a songwriting aid but it allows you to build "multi tracked" arrangements from a wide range of predefined parts. Some of the bass lines are very good. It's worth checking out, especially since there's a free "lite" version with most of the features active.
Try Virtual Bassist from Steinberg.