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This is not what Rules are made for but the following works well for simple cases:

solution[[All, 2]]

{1, 10, 100}

More "proper" way:

a /@ Range[3] /. solution

or equivalent suggested by Mr. Wizard

Array[a, 3] /. solution

For more nested list of rules you can use Cases (but then you have to keep an eye on what is what ;)) :

Cases[solution, HoldPattern[a[_] -> x_] :> x, Infinity]

{1, 10, 100}

Or the same but in different form:

Cases[solution, Rule[a[_], x_] :> x, Infinity]
Cases[solution, (a[_] -> x_):> x, Infinity]

Remarks about Cases:

• For your example, default levelspec for Cases, which is {1}, will do the job.
• ReplaceAll (/.) will work with solution containing Rule (->) or RuleDelayed (:>) but for Cases we have to point this out:

solution = {a[1] -> 1, a[2] :> 5, a[3] -> 100}

Cases[solution, Rule[a[_], x_] :> x]
Cases[solution, (Rule | RuleDelayed)[a[_], x_] :> x]

{a[1] -> 1, a[2] :> 5, a[3] -> 100}
{1, 100}
{1, 5, 100}

This is not what Rules are made for but the following works well for simple cases:

solution[[All, 2]]

{1, 10, 100}

More "proper" way:

a /@ Range[3] /. solution

or equivalent suggested by Mr. Wizard

Array[a, 3] /. solution

For more nested list of rules you can use Cases (but then you have to keep an eye on what is what ;)) :

Cases[solution, (a[_] -> x_):> x, Infinity]

{1, 10, 100}

Or the same but in different form:

Cases[solution, Rule[a[_], x_] :> x, Infinity]
Cases[solution, HoldPattern[a[_] -> x_] :> x, Infinity]

Remarks about Cases:

• For your example, default levelspec for Cases, which is {1}, will do the job.
• ReplaceAll (/.) will work with solution containing Rule (->) or RuleDelayed (:>) but for Cases we have to point this out:

solution = {a[1] -> 1, a[2] :> 5, a[3] -> 100}

Cases[solution, Rule[a[_], x_] :> x]
Cases[solution, (Rule | RuleDelayed)[a[_], x_] :> x]

{a[1] -> 1, a[2] :> 5, a[3] -> 100}
{1, 100}
{1, 5, 100}

This is not what Rules are made for but the following works well for simple cases:

solution[[All, 2]]

{1, 10, 100}

More "proper" way:

a /@ Range[3] /. solution

or equivalent suggested by Mr. Wizard

Array[a, 3] /. solution

For more nested list of rules you can use Cases (but then you have to keep an eye on what is what ;)) :

Cases[solution, HoldPattern[a[_] -> x_] :> x, Infinity] (*here levelspec 1 will work too*)

{1, 10, 100}

Or the same but in different form:

Cases[solution, Rule[a[_], x_] :> x, Infinity] (*also for your case levelspec 1 will do*)

This is not what Rules are made for but the following works well for simple cases:

solution[[All, 2]]

{1, 10, 100}

More "proper" way:

a /@ Range[3] /. solution

or equivalent suggested by Mr. Wizard

Array[a, 3] /. solution

For more nested list of rules you can use Cases (but then you have to keep an eye on what is what ;)) :

Cases[solution, HoldPattern[a[_] -> x_] :> x, Infinity]

{1, 10, 100}

Or the same but in different form:

Cases[solution, Rule[a[_], x_] :> x, Infinity]
Cases[solution, (a[_] -> x_):> x, Infinity]

Remarks about Cases:

• For your example, default levelspec for Cases, which is {1}, will do the job.
• ReplaceAll (/.) will work with solution containing Rule (->) or RuleDelayed (:>) but for Cases we have to point this out:

solution = {a[1] -> 1, a[2] :> 5, a[3] -> 100}

Cases[solution, Rule[a[_], x_] :> x]
Cases[solution, (Rule | RuleDelayed)[a[_], x_] :> x]

{a[1] -> 1, a[2] :> 5, a[3] -> 100}
{1, 100}
{1, 5, 100}

This is not what Rules are made for but the following works well for simple cases:

solution[[All, 2]]

{1, 10, 100}

More "proper" way:

a /@ Range[3] /. solution

{1, 10, 100}

For more nested list of rules you can use Cases (but then you have to keep an eye on that is what ;)) :

Cases[solution, HoldPattern[a[_] -> x_] :> x, Infinity] (*here levelspec 1 will work too*)

{1, 10, 100}

Or the same but in different form:

Cases[solution, Rule[a[_], x_] :> x, Infinity] (*also for your case levelspec 1 will do*)

{1, 10, 100}

This is not what Rules are made for but the following works well for simple cases:

solution[[All, 2]]

{1, 10, 100}

More "proper" way:

a /@ Range[3] /. solution

or equivalent suggested by Mr. Wizard

Array[a, 3] /. solution

For more nested list of rules you can use Cases (but then you have to keep an eye on what is what ;)) :

Cases[solution, HoldPattern[a[_] -> x_] :> x, Infinity] (*here levelspec 1 will work too*)

{1, 10, 100}

Or the same but in different form:

Cases[solution, Rule[a[_], x_] :> x, Infinity] (*also for your case levelspec 1 will do*)